Health monitoring of aircraft landing gear mechanical structures

ABSTRACT

Methods and systems are provided that facilitate the maintenance of levered landing gears by monitoring the condition of the stop pads of such landing gears. One embodiment provides for calibrating a sensor for measuring a condition of a stop joint formed by a first stop pad and a second stop pad of a levered landing gear against a nominal condition of at least one of the first stop pad and the second stop pad; monitoring, by the sensor, a current condition of the at least one of the first stop pad and the second stop pad from the nominal condition; determining whether a non-conformance from the nominal condition of the at least one of the first stop pad and the second stop pad has been detected by the sensor for the current condition; and in response to determining that the non-conformance has been detected, generating an alert.

FIELD

Aspects of the present disclosure provide improved monitoring forlanding gear systems and improvements to the aircraft that employ suchlanding gear systems.

BACKGROUND

The present invention relates to landing gear systems, and morespecifically, to levered landing gear systems. A levered landing gear isdesigned to travel between an extended position and a static positionduring takeoff and landing operations, and will remain in the staticposition to support the weight of the aircraft while resting on theground or taxiing. When in the static position, paired stop pads—atleast one strut or “upper” pad and at least one truck or “lower”pad—come into contact with one another to arrest the travel of thelanding gear from the extended position. Arresting this travel requiresthe stop pads, and the levered landing gear as a whole, to carrysignificant static, dynamic, and impact loads to support the aircraftwhile on the ground. These stop pads are in typically contact with oneanother while the aircraft is on the ground, and are generally notvisible to maintenance and service personnel unless the weight of theaircraft is shifted off of the levered landing gear, which is anexpensive and time consuming operation that is often conducted asscheduled maintenance.

SUMMARY

The present disclosure provides in one embodiment levered landing gear,comprising: a first shock strut having a longitudinal axis, the firstshock strut including first stop pad having a first contact surface; asecond shock strut disposed concentrically with the first shot strutalong the longitudinal axis such that the first shock strut and thesecond shock strut extend along a shared extension axis; a truck levercoupled to the first shock strut and the second shock strut such thatthe second shock strut pivots the truck lever relative to the firstshock strut, the truck lever including a second stop pad having a secondcontact surface, wherein the truck lever pivots between an extendedposition and a static position relative to the first shock strut, andwherein the first contact surface makes physical contact with the secondcontact surface when the truck lever is in the static position to form astop joint for the levered landing gear; and a first sensor disposedwith the levered landing gear and configured to measure a firstcondition of the stop joint.

In one aspect, in combination with any example above or below, the firstsensor is a camera disposed of in the first stop pad, and the firstcondition of the stop joint measured by the camera includes at least oneof: a debris presence in the stop joint and a surface condition of thesecond contact surface.

In one aspect, in combination with any example above or below, thecamera is a three-dimensional camera mounted in first stop pad via acamera spring and includes a light oriented to shine on the second stoppad while the levered landing gear is in the extended position.

In one aspect, in combination with any example above or below, the firstsensor is a gap sensor disposed of in the first stop pad, and the firstcondition of the stop joint measured by the gap sensor includes a gapdistance between at least a portion of the first contact surface and atleast a portion of the second contact surface.

In one aspect, in combination with any example above or below, the gapsensor is an eddy current sensor mounted in a fixed position within thefirst stop pad.

In one aspect, in combination with any example above or below, the firstsensor is a thickness sensor disposed of in the second stop pad, and thefirst condition of the stop joint measured by the thickness sensorincludes a thickness of the second stop pad.

In one aspect, in combination with any example above or below, thethickness sensor is an ultrasound sensor held in contact with a secondmounting surface of the stop pad via a spring, the second mountingsurface opposite to the second contact surface of the second stop pad.

In one aspect, in combination with any example above or below, thelevered landing gear further comprises a second sensor disposed with thelevered landing gear that measures a second condition of the stop joint.

In one aspect, in combination with any example above or below, the firstsensor is a camera mounted on the first shock strut, and the firstcondition of the stop joint measured by the camera includes at least oneof: a debris presence in the stop joint and a surface condition of thesecond contact surface; and the levered landing gear further includes asecond sensor, wherein the second sensor is a gap sensor disposed of inthe first stop pad, and the second condition of the stop joint measuredby the gap sensor includes a gap distance between at least a portion ofthe first contact surface and at least a portion of the second contactsurface.

In one aspect, in combination with any example above or below, the firstsensor is a camera disposed of in the first stop pad, and the firstcondition of the stop joint measured by the camera includes at least oneof: a debris presence in the stop joint and a surface condition of thesecond contact surface; and the levered landing gear further includes asecond sensor, wherein the second sensor is a thickness sensor disposedof in the second stop pad, and wherein the second condition of the stopjoint measured by the thickness sensor includes a thickness of thesecond stop pad.

In one aspect, in combination with any example above or below, thelevered landing gear further comprises a computing device incommunication with the first sensor that compares the measured conditionagainst a threshold and, in response to the measured conditionsatisfying the threshold, generates an alert associated with themeasured condition.

The present disclosure provides in another embodiment, a stop pad in alevered landing gear, comprising: a first contact surface, disposed ofin the levered landing gear relative to a second contact surface of thelevered landing gear so as to form a stop joint for a static position ofthe levered landing gear with the second contact surface; and a firstsensor disposed of within the stop pad, wherein the first sensormeasures a condition of the stop joint.

In one aspect, in combination with any example above or below, the firstsensor is a camera sensor focused on the second contact surface, and thecondition of the stop joint is measured according to a visual appearanceof the second contact surface.

In one aspect, in combination with any example above or below, the firstcontact surface is defined by a truck pad of the levered landing gear,wherein the first sensor is a gap sensor, and wherein the condition is agap distance between the first contact surface and the second contactsurface while the levered landing gear is in a static position.

In one aspect, in combination with any example above or below, a secondsensor of a camera sensor is focused on the second contact surfacedefined in a strut pad of the levered landing gear, wherein the secondsensor is signaled to capture of an image of the second contact surfacewhile the levered landing gear is in an extended position and correlatethe image with the gap distance measured while the levered landing gearis in the static position.

In one aspect, in combination with any example above or below, the firstcontact surface is defined in a truck pad of the levered landing gear,wherein the first sensor is a thickness sensor, and wherein thecondition is a pad thickness of the truck pad.

In one aspect, in combination with any example above or below, a secondsensor of a camera sensor is focused on the first contact surface,wherein the second sensor is signaled to capture of an image of thefirst contact surface while the levered landing gear is in an extendedposition and correlate the image with the pad thickness measured by thethickness sensor.

In a further embodiment, the present disclosure provides a method formonitoring the health of a levered landing gear, comprising: calibratinga sensor for measuring a condition of a stop joint formed by a firststop pad and a second stop pad of the levered landing gear against anominal condition of at least one of the first stop pad and the secondstop pad; monitoring, by the sensor, a current condition of the at leastone of the first stop pad and the second stop pad from the nominalcondition; determining whether a non-conformance from the nominalcondition of the at least one of the first stop pad and the second stoppad has been detected by the sensor for the current condition; and inresponse to determining that the non-conformance has been detected,generating an alert.

In one aspect, in combination with any example above or below,monitoring the health of a levered landing gear further comprises, inresponse to determining that the non-conformance has been detected:capturing an image via a camera focused on the at least one of the firststop pad and the second stop pad; storing the image for retrieval tocorroborate the determination that the non-conformance has beendetected; and displaying the image to a user in response to the useracknowledging the alert.

In one aspect, in combination with any example above or below, thecondition monitored includes at least one of: a thickness of the secondstop pad measured by the sensor via ultrasound; a gap distance betweenthe first surface and the second surface measured by the sensor via aneddy current; a gap distance between the first surface and the secondsurface measured by the sensor via range finding; a visual indication ofdebris in the stop joint measured by the sensor via image recognition; avisual indication of corrosion in the stop joint measured by the sensorvia image recognition; and a visual indication of surface marring in thestop joint measured by the sensor via image recognition.

BRIEF DESCRIPTION OF THE DRAWINGS

So that the manner in which the above recited features of the presentdisclosure can be understood in detail, a more particular description ofthe disclosure, briefly summarized above, may be had by reference toaspects, some of which are illustrated in the appended drawings.

FIGS. 1A-C illustrate various views of an example levered landing gearaccording to embodiments of the present disclosure.

FIG. 2 illustrates a detailed view of a camera sensor integrated in anexample levered landing gear according to embodiments of the presentdisclosure.

FIG. 3 illustrates a detailed view of a gap sensor integrated in anexample levered landing gear according to embodiments of the presentdisclosure.

FIG. 4 illustrates a detailed view of a thickness sensor integrated inan example levered landing gear according to embodiments of the presentdisclosure.

FIG. 5 illustrates an example deployment of a single camera sensor andmultiple gap sensors integrated in an example levered landing gearaccording to an embodiment of the present disclosure.

FIG. 6 illustrates an example deployment of a single camera sensor andmultiple thickness sensors integrated in an example levered landing gearaccording to an embodiment of the present disclosure.

FIG. 7 illustrates an example deployment of multiple camera sensors andmultiple thickness sensors integrated in an example levered landing gearaccording to an embodiment of the present disclosure.

FIG. 8 is a flowchart illustrating the general steps in an examplemethod for providing health monitoring of aircraft landing gearmechanical structures according to embodiments of the presentdisclosure.

DETAILED DESCRIPTION

The present disclosure relates to apparatuses and methods thatfacilitate the maintenance of levered landing gears by monitoring thecondition of the stop pads of such landing gears. An aircraft withlevered landing gear employs the landing gear in an extended position togive additional height to the landing gear compared to the staticposition to thereby affect the angles of attack possible during takeoffand/or landing. While that aircraft is on the ground (e.g., resting,taxiing), the levered landing gear of the aircraft are in a staticposition in which the stop pads that support the weight of the aircraftare subject to various static, dynamic, and impact loads. The contactingsurfaces of the stop pads, as well as other portions of the stop pads,are not visible while in the static position, which prevents maintenanceand service personnel from inspecting the stop pads. To inspect the stoppads, the aircraft must be lifted off of its landing gear so that thelevered landing gear may travel to an extended position in which thestop pads are visible. As will be appreciated, lifting an aircraft takesthe aircraft out of service for a significant period of time, and thuscan be very costly to the operators of the aircraft.

To reduce the costs associated with employing levered landing gears andthe servicing thereof, the stop pads are often inspected on a scheduledbasis (allowing the downtime to be predictable and co-scheduled withother maintenance) and are made of robust materials to extend the lifeof the stop pads past the scheduled maintenance periods. As will beappreciated, scheduled maintenance may result in premature replacementof parts, such as a pad with a five-year operation life but a three-yearinspection schedule being replaced after only three years of operation.As will also be appreciated, the reliance on scheduled maintenance canlead to over-engineered parts being used, which increase the cost of thelanding gear and result in heavier landing gear assemblies (affectingcost of the aircraft and the fuel efficiency of the aircraft in turn).

The present disclosure presents systems and methods to inspect the stoppads outside of the scheduled maintenance periods, thus enablingconditional maintenance to be performed in addition to the scheduledinspection and/or servicing of the levered landing gears. Varioussensors are included in the stops pads and the levered landing gearassemblies that monitor the health of the stop pads and alertmaintenance personnel when a non-conformance from the operationalprofile of the stop pad is noticed. Depending on the sensors used,non-conformances include the thicknesses of the pads, the surfaceconditions of the pads, whether the paired stop pads are makingconsistent contact with one another, and whether debris is present onthe pads.

FIGS. 1A-C illustrate various views of an example levered landing gear100 to highlight the operation of the stop pads that will be monitoredaccording to embodiments of the present disclosure. FIG. 1A illustratesa lateral cutaway view of an example levered landing gear 100 in theextended position, while FIG. 1B illustrates a lateral cutaway view ofthat example levered landing gear 100 in the static position. FIG. 1Cillustrates an isometric view of an example levered landing gear 100 inthe extended position. Depending on the view presented, a givencomponent may be fully or partially obscured by another component.Therefore, one or more portions of a given component may be labeled withthe same reference number for ease in identifying a given component inthe illustrated views. Multiple instances of a given component aredifferentiated from one another by the use of a letter in conjunctionwith the reference number. One of ordinary skill in the art willappreciate that the illustrations provide a set of non-limiting examplesof some of the various shapes, dimensions, and arrangements possible fora levered landing gear 100 and its components.

For ease of understanding, the current disclosure focuses itsexplanation of a first shock strut 110, a second shock strut 120, atruck lever 130, a connecting link 140, a pivot joint 150, a strut pad160, a truck pad 170, the stop joint 180 formed by the strut pad 160 andthe truck pad 170, a landing gear axle 190, and various sensors for usewith the levered landing gear, although one of ordinary skill in the artwill appreciate that additional components are included in a leveredlanding gear 100 that are not discussed in detail herein. Collectively,the strut pad 160 and the truck pad 170 may be referred to as “stoppads,” and either the strut pad 160 or the truck pad 170 may be referredto as a “first” stop pad and the other referred to as a “second” stoppad. Similarly, the various features of the stop pads may be referred toby the ordinal designators used with the associated stop pad, such as,for example, a “first contact surface” referring to the contact surfaceof the first stop pad (either the strut pad 160 or the truck pad 170).

The first shock strut 110 has a longitudinal axis having a first end111, which connects with the aircraft, and a second end 112, whichpivotally connects with the truck lever 130 via a first pivot joint 150a. The first shock strut 110, at or near the second end 112, includesthe strut pad 160, which has a contact surface 161. Additional referencewill be made to a mounted surface 162 of the strut pad 160, which is incontact with the body of the first shock strut 110, and is not visiblein certain illustrations of the embodiments. Similarly, a strut padthickness 163 is defined by the distance between the contact surface 161and the mounted surface 162. In various embodiments, the strut pad 160and the surrounding structure of the first shock strut 110 define one ormore cavities in which various sensors are disposed. In someembodiments, the various sensors extend through the strut pad 160 or areassociated with a through-hole in the contact surface 161.

The second shock strut 120 is disposed concentrically with the firstshock strut 110, and shares a longitudinal axis with the first shockstrut 110. A first end 121 of the second shock strut 120 is locatedwithin a cavity defined by the first shock strut 110, and the positionof the first end 121 of the second shock strut 120 will vary as thesecond shock strut 120 extends to and from the extended position and thestatic position along the longitudinal axis shared with the first shockstrut 110. The second end 122 of the second shock strut 120 is coupledto the truck lever 130 via a connection link 140. The connection link140 is connected with the second shock strut 120 via a second pivotjoint 150 b and is connected with the truck lever 130 via a third pivotjoint 150 c.

The truck lever 130 is coupled to the first shock strut 110 and thesecond shock strut 120 such that the second shock strut 120, as itextends to and from the extended and static positions, pivots the trucklever 130 relative to the first shock strut 110. The truck lever iscoupled at a first end 131 with the first shock strut 110 via the firstpivot joint 150 a and is coupled at a second end 132 with a landing gearaxle 190, by which the wheels may be mounted with the levered landinggear 100.

The truck lever 130 includes the truck pad 170, which has a contactsurface 171, configured to interface with the contact surface 161 of thestrut pad 160 and thereby form the stop joint 180 when the truck lever130 is in the static position. Additional reference will be made to amounted surface 172 of the truck pad 170, which is in contact with thebody of the truck lever 130, and is not visible in certain illustrationsof the embodiments.

The various sensors used in one or more embodiments of the presentdisclosure are disposed with the levered landing gear 100 at variouspositions to measure conditions of the stop joint 180 to ensure thehealth of the mechanical components of the levered landing gear 100. Thetypes of sensors; where those sensors are disposed of in the leveredlanding gear 100; the conditions of the stop joint 180 that aremeasured; whether a first sensor is used alone, part of a sensor array,or jointly with a second sensor; etc.; may vary in one or moreembodiments of the present disclosure.

In various embodiments, the materials from which the strut pad 160 isconstructed may be the same as or vary from the materials used for thefirst shock strut 110. Similarly, the materials from which the truck pad170 is constructed may be the same as or vary from those materials usedin the truck lever 130. In various embodiments, each of the strut pad160 and the truck pad 170 are replaceable elements separable from therespective first shock strut 110 and the truck lever 130, although insome embodiments one or more of the strut pad 160 and the truck pad 170may be designated regions of the first shock strut 110 or the trucklever 130, respectively. In some embodiments, the material that thestrut pad 160 is made of is a harder material than the material than thetruck pad 170 is made of, such as, for example, a steel versus a bronze(e.g., CuNiSn), so that wear can be observed more readily in one of thepads.

The stop joint 180 is formed by the strut pad 160 and the truck pad 170when the landing gear 100 is in the static position. The contactingsurfaces of the strut pad 160 and the truck pad 170 (e.g., the contactsurface 161 and the contact surface 171) come into contact and help bearthe weights and forces applied by the aircraft to the wheels. Over time,wear on the contacting surfaces will erode one or more of the strut pad160 and the truck pad 170, contaminants may corrode one or more of thecontact surfaces, and cracks, scratches, gouges, etc. may form.Additionally, various debris and contaminants may be introduced into thestop joint 180, which may damage the one or more of the strut pad 160and the truck pad 170 if not removed.

In the extended position, a levered landing gear 100 provides additionalheight for the aircraft relative to the static position. As the secondshock strut 120 extends, the connecting link 140 translates the verticalmotion of the second shock strut 120 to pivot the truck lever 130 aboutthe first pivot joint 150 a. As shown in the illustrated embodiments,the third pivot joint 150 c is located between the first pivot joint 150a and the landing gear axle 190 so that as the second shock strut 120extends to the extended position, additional height is added to thelevered landing gear 100. The amount of height added to the leveredlanding gear 100 roughly corresponds to the distance between the firstpivot joint 150 a and the landing gear axle 190. As the levered landinggear 100 extends to the extended position, the stop joint 180 opens,moving the strut pad 160 and the truck pad 170 away from one another andexposing the respective contact surfaces (i.e., contact surface 161 andthe contact surface 171).

As will be appreciated, the contacting surfaces of the stop joint 180are obscured by one another when the levered landing gear 100 is in thestatic position, rendering maintenance personnel unable to visuallyinspect the contact surface 161 of the strut pad 160 or the contactsurface 171 of the truck pad 170. However, the levered landing gear 100is not typically in the extended position, where the contact surfacesare visible, except during takeoff and landing. Therefore, formaintenance personnel to view the contact surfaces, the aircraft islifted or jacked to remove its weight from the landing gear 100 so thatthe levered landing gear 100 may be placed in the extended position forvisual inspection of the contact surfaces. These “lifting” or “jacking”operations are typically conducted on a scheduled basis (e.g., every Xflight hours, every Y months) as the operations take the aircraft out ofservice for extended periods of time. The various sensors discussed inthe present disclosure provide opportunities to evaluate the health ofthe stop joint 180 in addition to the scheduled maintenance inspections,and several example sensors are discussed in greater detail in regard toFIGS. 2-4.

FIG. 2 illustrates a detailed view 200 of a camera sensor 210 integratedin an example levered landing gear 100 according to an embodiment of thepresent disclosure. A camera sensor 210 is focused on some or all of acontact surface of a stop pad and is configured or operable to measure avisual condition of a contact surface and/or to provide images of thecontact surface to maintenance personnel to corroborate the findings ofany other sensors in use in the levered landing gear 100. Although FIG.2 illustrates one camera sensor 210 integrated in the strut pad 160 andthe first shock strut 110, it will be appreciated that a camera sensor210 may also be integrated in the truck pad 170 and the truck lever 130,or in a mount external to both the strut pad 160 and the truck pad 170,and that more than one camera sensor 210 may be employed in an array.

The camera sensor 210 includes a digital camera that is in communicationwith a computing device 240. In various embodiments, the camera sensor210 receives power via camera cabling 230, which may also be used tocommunicate the images captured by the camera sensor 210 to thecomputing device 240. In other embodiments, the camera cabling 230 mayalso be used to communicate with an external computing device, such as adiagnostic terminal.

In various embodiments, the camera sensor 210 is a camera (or array ofcameras) capable of capturing three-dimensional (3D) images, althoughtwo-dimensional (2D) camera sensor 210 may also be employed. The camerasensor 210 includes an appropriate lens to focus the camera sensor 210on the desired portion or portions of the contact surfaces to monitor(e.g., a wide angle lens, a multi-focal lens). The camera sensor 210 isable to automatically focus, and thus is able to capture images of thecontact surface at various stages of extension between the extended andstatic positions of the levered landing gear 100 as the contact surfacesapproach or recede from one another. In some embodiments, the camerasensor 210 includes a light source that may be activated or deactivatedto ensure a consistent light level in the images of the contact surfacescaptured by the camera sensor 210.

The camera sensor 210 may be disposed of within one of the stop pads (tomonitor the surface conditions of the other stop pad) or externally tothe stop pads (to monitor the surface conditions of one or both stoppads).

When disposed internally to the stop pads, a cavity defined in the stoppad and/or the body of the associated shock strut holds the camerasensor 210 with its image capturing end facing outward. The shape andsize of the camera sensor 210 relative to the cavity and/or an openingdefined in the stop pad may capture the camera sensor 210 within thecavity so that the lens of the camera sensor 210 does not extend pastthe contacting surface of the stop pad that the camera sensor 210 ismounted within. To cushion the camera sensor 210 from impacts duringaircraft operations (e.g., landing gear extension/retraction, takeoff,landing), one or more camera actuator members 220, such as springs,pistons, micro linear actuators, or the like, are connected with thecamera sensor 210 within its housing.

When disposed externally to the stop pads, such as in a space betweenstop pads, the camera sensor 210 may be held in a fixed mount or arotating mount. An external mount for the camera sensor 210 may alsoinclude springs to cushion the camera from impacts. When employing arotating mount, the second shock strut 120 may be communicated to thecamera sensor 210 to co-rotate or counter-rotate the camera relative tothe truck lever 130 as the truck lever 130 rotates between the extendedand static positions to maintain or establish a field of view thatincludes the monitored contacting surfaces.

The computing device 240 to which the camera sensor 210 is connectedincludes computer readable memory storage devices (e.g., hard drives,RAM (Random Access Memory), flash drives) and processors that executelogic stored on the memory storage devices, which may control variouscomponents of the aircraft or interface to control or diagnose variousfeatures of the camera sensor 210. For example, the computing device 240may be the onboard computer for the aircraft or may be an externalcomputer used by maintenance personnel that is connected to the camerasensor 210 during maintenance checks and/or pre-/post-flight checkups.The camera sensor 210 may also include computer readable memory storagedevices and processors to control its capture and/or analysis of images,or may rely on the computing device 240 for control and/or storage.

In some embodiments, the camera sensor 210 stores one or more imagescaptured during takeoff and/or landing (when the contact surfaces arevisible) in an internal memory storage device or a memory storage deviceof the computing device 240. The camera sensor 210 may take images inresponse to a signal from the computer device 240 indicating that thesecond shock strut 120 is moving the levered landing gear 100 into orout of the extended position, a timing signal (e.g., every X seconds, Yseconds after an event), or a signal from another sensor indicating thata non-conformance has been detected by that sensor.

The captured images may be retained for human inspection, or compared bythe computing device 240 against a nominal state of the contact surfaceto generate alerts when a non-conformance is visually detected. Forexample, maintenance personnel may manually check the captured images todetermine whether maintenance should be performed on the levered landinggear 100. In another example, an artificial intelligence (AI) orcomputer learning algorithm may be employed by the computing device 240to learn when a human or another sensor would indicate anon-conformance, and generate alerts based on the learned visualappearances of non-conformances. In a further example, various imagethresholds may be used against the captured images, such as, forexample, a color or albedo threshold for the image that indicates apercentage of the image that must be within a predetermined range of anominal color or albedo to alert when a patina has been removed (e.g.,via a scratch or pitting revealing a differently colored/reflectivesubstrate), a contaminant/debris has been introduced (e.g., replacingthe expected color or reflectiveness with the contaminant'scolor/reflectiveness), or corrosion is occurring.

FIG. 3 illustrates a detailed view 300 of a gap sensor 310 integrated inan example levered landing gear 110 according to an embodiment of thepresent disclosure. Although FIG. 3 illustrates one gap sensor 310integrated in the strut pad 160 and the first shock strut 110, it willbe appreciated that a gap sensor 310 may also be integrated in the truckpad 170 and the truck lever 130, and that more than one gap sensor 310may be employed in an array. When employed in an array, each gap sensor310 of the array may measure a localized portion of the gap distance 181in the stop joint 180 or may be combined to measure the gap distance 181as an average across several locations.

In various embodiments, the gap sensor 310 includes one or more of aneddy current sensor or a range finder to determine a gap distance 181 inthe stop joint 180 between the gap sensor 310 and the opposing stoppad's contact surface. The gap sensor 310 is disposed of within the stoppad and surrounding substrate via a bushing 320 at a calibrated distancefrom an opposing stop pad, and communicated with the computing device240 via a gap cable 330. The bushing 320 and an associated locking nut340 hold the gap sensor 310 in a stable position relative to the nominalposition of the contact surfaces so that as the stop pads wear or erode,or as cracks or debris are introduced that alter the measured gapdistance 181, the change will be noted.

For example, when a through-hole in the strut pad 160 includes a gapsensor 310, the distance from the measurement point of the gap sensor310 and the truck pad 170 (i.e., the opposing stop pad in this example)is calibrated for the nominal gap distance 181 when the stop pads formthe stop joint 180. The measured distances between the gap sensor 310and the truck pad 170 are compared against a gap threshold, either bythe gap sensor 310 or the computing device 240 to which the gap sensor310 communicates its measurements, so that an alert is generated whenthe distance of the gap distance satisfies the gap threshold. In variousembodiments, the gap threshold specifies a range (positive and negativerelative to the nominal gap distance) that the gap distance may varybefore an alert is generated.

When the gap sensor 310 is an eddy current sensor or other inductivesensor, an alternating current energizes the gap sensor 310, whichinduces eddy current into the surrounding materials of one or more ofthe stop pads. These eddy currents, in turn, affect an impedance withinthe gap sensor 310 that is measured to determine a corresponding gapdistance 181 between the gap sensor 310 and the opposing contactsurface.

When the gap sensor 310 is a range finder, the gap sensor 310 generatesa ranging signal when activated, and measures a time for the rangingsignal to be reflected back to the range finder. Examples of rangefinders include laser range finders and acoustic range finders (e.g.,sonar, ultrasound) to determine the return times of the ranging signal(e.g., a laser or soundwave) through the speed of the ranging signalthrough the material forming the gap (e.g., air).

As will be appreciated, as material expand and contract due to changesin temperature, the temperature of the stop pads and the gap sensor 310may affect the measured gap distance. In various embodiments, thecomputing device 240 will apply a temperature offset to the measureddistances reported by the gap sensor 310 or alter the gap threshold toaccount for changes in temperature that will affect proper alerting(reducing false positives and false negatives). In other aspects, thegap sensor 310 includes temperature compensations so that as thetemperature of the gap sensor 310 and stop pads change, the reportedchange in gap distance 181 will be compensated for.

FIG. 4 illustrates a detailed view 400 of a thickness sensor 410integrated in an example levered landing gear 100 according toembodiments of the present disclosure. Although FIG. 4 illustrates onecamera sensor 210 integrated in the truck pad 170 and the truck lever130, it will be appreciated that a thickness sensor 410 may also beintegrated in the strut pad 160 and the first shock strut 110, and thatmore than one thickness sensor 410 may be employed in an array. Whenemployed in an array, each thickness sensor 410 may measure a localizedportion of an associated stop pad or may be combined to measure thethickness as an average across several locations.

In some embodiments, the thickness sensor 410 is an ultrasound sensorthat measures the thickness of the associated stop pad by generatingultrasonic sound waves that are transmitted into the mounted surface ofthe stop pad and measuring a time that it takes for the sound waves toreflect from the contact surface back to the thickness sensor 410.

The thickness sensor 410 is disposed internally to the stop pads, in acavity defined in the body of the truck lever 130 when associated withthe truck pad 170 or in the body of the first shock strut 110 whenassociated with the strut pad 160. The thickness sensor 410 is held incontact with the stop pad via a thickness actuator member 420, such as aspring, piston, micro linear actuator, or the like, which also serves tocushion the thickness sensor 410 from impacts during aircraft operations(e.g., landing gear extension/retraction, takeoff, landing).Measurements of the pad thickness are transmitted to the computingdevice 240 via the thickness cable 430, which may also supply power forthe thickness sensor 410.

For example, when the truck pad 170 includes a thickness sensor 410, thethickness sensor 410 is calibrated for the nominal pad thickness 173. Asthe truck pad 170 is used, wear and tear on the pad will erode thethickness thereof. Additionally, if any cracks or pitting develop in thetruck pad 170, the localized thickness will be lowered from the nominalpad thickness 173. Moreover, if the truck pad 170 is overly compressedor develops a patina, localized portions of the truck pad 170 may exceedthe nominal pad thickness 173. The measured pad thicknesses 173 arecompared against a thickness threshold, either by the thickness sensor410 or the computing device 240 to which the thickness sensor 410communicates its measurements, so that an alert is generated when thepad thickness satisfies the thickness threshold. In various embodiments,the thickness threshold specifies a range (positive and negativerelative to the nominal gap distance) that the pad thickness may varybefore an alert is generated.

For example, when employed to measure the thickness of the truck pad170, the thickness sensor 410 is held in contact with the second mountedsurface 172 of the truck pad 170 by the thickness actuator member 420.The thickness sensor 410 measures a pad thickness 173 of the truck pad170, defined by the distance between the mounted surface 172 and thecontact surface 171.

The thickness sensor 410 may measure the pad thickness when the leveredlanding gear 100 is in the extended position or on the static position.As will be appreciated, as the temperature of the truck pad 170 (or thestrut pad 160 if a thickness sensor 410 is employed therewith) changes,the thickness or the acoustic properties (e.g. the speed at which soundtravels through the material) of the stop pad may change. In variousembodiments, the computing device 240 will apply a temperature offset tothe measured pad thickness reported by the thickness sensor 410 or alterthe thickness threshold to account for changes in temperature that willaffect proper alerting (reducing false positives and false negatives).In other aspects, the thickness sensor 410 includes temperaturecompensations so that as the temperature of the thickness sensor 410 andstop pads change, the reported change in pad thickness will becompensated for when reported to the computing device 240.

FIG. 5 illustrates an example deployment 500 of a single camera sensor210 and multiple gap sensors 310 integrated in an example leveredlanding gear 110 according to an embodiment of the present disclosure.As will be appreciated, the example deployment 500 is provided as anon-limiting example of the present disclosure, and embodiments withmore, fewer, or different components that may be arranged in differentpositions are contemplated.

In the example deployment 500, a first gap sensor 310 a and a second gapsensor 310 b are disposed of in a first strut pad 160 a and a secondstrut pad 160 b, respectively. The first gap sensor 310 a measures adistance to a first truck pad 170 a as a first gap distance 181 a, andthe second gap sensor 310 b measures a distance to a second truck pad170 b as a second gap distance 181 b. As will be appreciated, each ofthe first strut pad 160 a, the second strut pad 160 b, the first truckpad 170 a, and the second truck pad 170 b may be individual stop padsthat are separately replaceable, or may be localized portions of asingle respective strut pad 160 or truck pad 170.

Both the first gap sensor 310 a and the second gap sensor 310 b arecommunicated to a computing device 240 via a respective first gap cable330 a and a second gap cable 330 b. The computing device 240 receivesthe measured gap distances 181 from the gap sensors 310, and may comparethe measured gap distances 181 individually or collectively against oneor more gap thresholds. For example, the computing device 240 mayaverage the readings received from the several gaps sensors 310 forcomparison against an average gap threshold and also compare theindividual readings received from specific gap sensors 310 againstlocalized gap thresholds.

In addition to the gap sensors 310, a camera sensor 210 is included inthe example deployment 500, which faces the truck pads 170 and isconnected with the computing device 240 via a camera cable 230. Invarious embodiments the camera sensor 210 is disposed of within thestrut pad 160, between individual strut pads 160 (e.g., a first strutpad 160 a and a second strut pad 160 b), or on the first shock strut 110separately from the stop pads. The camera sensor 210 may include a wideangle lens to focus on both truck pads 170 simultaneously, a movablelens to focus on the first truck pad 170 a and the second truck pad 170b at different times.

The computing device 240 may signal one or more of the sensors to takemeasurements at specific times, in response to specific conditions, ormay sample measurements when specific conditions are true. For example,the computing device 240 may signal the gap sensors 310 to take readingsof the gap distances 181 when the levered landing gear 100 enters thestatic position and every X seconds thereafter. In another example, thecomputing device 240 may signal the camera sensor 210 to capture imagesin response to the gap sensors 310 indicating a gap distance 181 that isout of conformance once the levered landing gear 100 transitions to anextended position. In a further example, the gap sensors 310 may takeconstant measurements, and the computing device 240 may ignore ordiscard data while the levered landing gear 100 is in the extendedposition. In another example, the camera sensor 210 captures severalimages while the levered landing gear 100 is in the extended position,and the computing device 240 determines whether to retain those imagesbased on whether the gap sensors 310 indicate that the gap distance 181is out of conformance when the levered landing gear 100 next returns tothe static position.

FIG. 6 illustrates an example deployment 600 of a single camera sensor210 and multiple thickness sensors 410 integrated in an example leveredlanding gear 110 according to an embodiment of the present disclosure.As will be appreciated, the example deployment 600 is provided as anon-limiting example of the present disclosure, and embodiments withmore, fewer, or different components that may be arranged in differentpositions are contemplated.

In the example deployment 600, a first thickness sensor 410 a and asecond thickness sensor 410 b are disposed of in a first truck pad 170 aand a second truck pad 170 b, respectively. The first thickness sensor410 a measures a thickness of the first truck pad 170 a as a first padthickness distance 173 a, and the second thickness sensor 410 b measuresa thickness of a second truck pad 170 b as a second thickness 173 b. Aswill be appreciated, each of the first strut pad 160 a, the second strutpad 160 b, the first truck pad 170 a, and the second truck pad 170 b maybe individual stop pads that are separately replaceable, or may belocalized portions of a single respective strut pad 160 or truck pad170.

Both the first thickness sensor 410 a and the second thickness sensor410 b are communicated to a computing device 240 via a respective firstthickness cable 430 a and a second thickness cable 430 b. The computingdevice 240 receives the measured thicknesses distances 173 from thethickness sensors 410, and may compare the measured thicknesses 173individually or collectively against one or more thickness thresholds.For example, the computing device 240 may average the readings receivedfrom the several thickness sensors 410 for comparison against an averagethickness threshold and also compare the individual readings receivedfrom specific thickness sensors 410 against localized thicknessthresholds.

In addition to the thickness sensors 410, a camera sensor 210 isincluded in the example deployment 600, which faces the truck pads 170and is connected with the computing device 240 via a camera cable 230.In various embodiments the camera sensor 210 is disposed of within thestrut pad 160, between individual strut pads 160 (e.g., a first strutpad 160 a and a second strut pad 160 b), or on the first shock strut 110separately from the stop pads. The camera sensor 210 may include a wideangle lens to focus on both truck pads 170 simultaneously, a movablelens to focus on the first truck pad 170 a and the second truck pad 170b at different times.

The computing device 240 may signal one or more of the sensors to takemeasurements at specific times, in response to specific conditions, ormay sample measurements when specific conditions are true. For example,the computing device 240 may signal the thickness sensors 410 to takereadings of the thicknesses 173 every X seconds. In another example, thecomputing device 240 may signal the camera sensor 210 to capture imagesin response to the thickness sensors 410 indicating a thickness 173 thatis out of conformance once the levered landing gear 100 transitions toan extended position. In a further example, the thickness sensors 410may take constant measurements, and the computing device 240 may samplethose measurements every X seconds. In another example, the camerasensor 210 captures several images while the levered landing gear 100 isin the extended position, and the computing device 240 determineswhether to retain those images based on whether the thickness sensors410 indicate that the thickness 173 is out of conformance.

FIG. 7 illustrates an example deployment 700 of multiple camera sensors210 and multiple thickness sensors 410 integrated in an example leveredlanding gear 110 according to an embodiment of the present disclosure.As will be appreciated, the example deployment 700 is provided as anon-limiting example of the present disclosure, and embodiments withmore, fewer, or different components that may be arranged in differentpositions are contemplated.

In the example deployment 700, a first thickness sensor 410 a and asecond thickness sensor 410 b are disposed of in a first truck pad 170 aand a second truck pad 170 b, respectively. The first thickness sensor410 a measures a thickness of the first truck pad 170 a as a first padthickness distance 173 a, and the second thickness sensor 410 b measuresa thickness of a second truck pad 170 b as a second thickness 173 b. Aswill be appreciated, each of the first strut pad 160 a, the second strutpad 160 b, the first truck pad 170 a, and the second truck pad 170 b maybe individual stop pads that are separately replaceable, or may belocalized portions of a single respective strut pad 160 or truck pad170.

Both the first thickness sensor 410 a and the second thickness sensor410 b are communicated to a computing device 240 via a respective firstthickness cable 430 a and a second thickness cable 430 b. The computingdevice 240 receives the measured thicknesses distances 173 from thethickness sensors 410, and may compare the measured thicknesses 173individually or collectively against one or more thickness thresholds.For example, the computing device 240 may average the readings receivedfrom the several thickness sensors 410 for comparison against an averagethickness threshold and also compare the individual readings receivedfrom specific thickness sensors 410 against localized thicknessthresholds.

In addition to the thickness sensors 410, a first camera sensor 210 aand a second camera sensor 210 b are included in the example deployment700, which faces the truck pads 170 and are connected with the computingdevice 240 via a first camera cable 230 a and a second camera cable 230b, respectively. Each of the camera sensors 210 are disposed of in arespective first strut pad 160 a and a second strut pad 160 b and eachcamera sensor 210 may include a wide angle lens to focus on both truckpads 170 simultaneously, a movable lens to focus on the first truck pad170 a and the second truck pad 170 b at different times, or may befocused on an individual truck pad 170. In embodiments where each camerasensor 210 is focused on both truck pads 170, the computing device maycombine the received images to create a 3D composite view of the contactsurfaces 171 of the truck pads 170.

The computing device 240 may signal one or more of the sensors to takemeasurements at specific times, in response to specific conditions, ormay sample measurements when specific conditions are true. For example,the computing device 240 may signal the thickness sensors 410 to takereadings of the thicknesses 173 every X seconds. In another example, thecomputing device 240 may signal the camera sensor 210 to capture imagesin response to the thickness sensors 410 indicating a thickness 173 thatis out of conformance once the levered landing gear 100 transitions toan extended position. In a further example, the thickness sensors 410may take constant measurements, and the computing device 240 may samplethose measurements every X seconds. In another example, the camerasensor 210 captures several images while the levered landing gear 100 isin the extended position, and the computing device 240 determineswhether to retain those images based on whether the thickness sensors410 indicate that the thickness 173 is out of conformance.

FIG. 8 is a flowchart illustrating the general steps in an examplemethod 800 for providing health monitoring of aircraft landing gearmechanical structures according to embodiments of the presentdisclosure. Method 800 begins at block 810, where one or more sensorsare calibrated for the nominal stop pad conditions that they aredesigned to monitor. For example, when one or more stop pads are newlyinstalled (replacing prior stop pads or at an initial installation) oneor more of a camera sensor 210, gap sensor 310, and thickness sensor 410are calibrated for an initial color/appearance, gap distance 181, and/orpad thickness. Calibration may involve adjusting the measurementsproduced by the sensors to match the nominal values, training a machinelearning algorithm to detect aberrant conditions based on a supervisedset of measurement data (e.g., prior known conforming and non-conformingmeasurements), and adjusting the various thresholds that the measuredvalues are compared against.

At block 820 the stop pads are monitored by the calibrated sensors. Invarious embodiments, the sensors may periodically (e.g., every Xseconds) take measurements of the stop pads, may take measurements inresponse to a user request for the signal, take measurements in responseto a transition to or from the extended position, or may takemeasurements constantly. The camera sensors 210 (if included) captureimages that are compared against nominal or initial image featurethresholds when the levered landing gear 100 is not in the staticposition. The gap sensors 310 (if included) take measurements of the gapdistance 181 when the levered landing gear 100 is in the staticposition. The thickness sensors 410 (if included) take measurements ofthe thickness of an associated stop pad regardless of the position ofthe levered landing gear 100. As will be appreciated, the variousmeasurements taken at block 820 may be measured across a period of timefor a given sensor, across an array of sensors of a given type, or crosscorrelated between sensors/arrays of sensors of different types. Forexample, an image taken during landing by a camera sensor 210 may becorrelated with the gap measurements taken by an array of gap sensors310 prior to landing and/or after landing when the levered landing gear100 is again in the static position.

Proceeding to block 830, it is determined, based on the measurements andthe associated thresholds, whether a non-conformance in the stop joint180 has been detected. A non-conformance is determined to have beendetected in response to a measurement from a sensor satisfying anassociated threshold. For example, a thickness measured above an upperthickness threshold or measured below a lower thickness threshold may bedetermined to satisfy a thickness threshold. In another example, gapdistance measured above an upper gap threshold or measured below a lowergap threshold may be determined to satisfy a gap threshold. In a furtherexample, a color or albedo of a captured image is compared against acolor or albedo threshold, and an image that has a calculated color oralbedo outside of the color or albedo threshold is determined to satisfythat threshold. In an additional example, a confidence of an imagerecognition system (using a calibrated AI or a machine learningalgorithm) that a non-conformance such as debris or damage to thecontact surfaces is present is compared against a confidence thresholdsuch that when the confidence exceeds the confidence threshold it isdetermined that the threshold has been satisfied.

In response to determining that a non-conformance has not been detected,method 800 returns to block 820. In response to determining that anon-conformance has been detected, method 800 optionally proceeds toblock 840 and then to block 850. In some embodiments, method 800continues performing blocks 820 and 830 once a non-conformance has beendetected to determine whether the non-conformance is transient,spreading, or can be verified by additional sensors.

Optionally, at block 840, camera sensor 210 is used to corroborate thedetected non-conformance. As will be appreciated, method 800 may foregoblock 840 in embodiments that do not include a camera sensor 210 or whenthe non-conformance is detected after the levered landing gear 100 hasentered the static position. The camera sensor 210 is signaled tocapture an image of the stop pads that is associated with the detectednon-conformance so that maintenance personnel may verify the presence ofthe indicated non-conformance and/or so that an image recognition systemmay be trained to identify non-conformances from images of the contactsurfaces. The captured image is then stored in the camera sensor 210and/or the computing device 240 for retrieval to corroborate thedetermination that the non-conformance has been detected.

At block 850 an alert is generated in response to the detectednon-conformance. In various embodiments, the alert is generated and/ordisplayed by the computing device 240. When block 840 is performed, thealert may be displayed along with the image of the detectednon-conformance or in response to maintenance personnel or an operatorof the aircraft acknowledging the alert (e.g., via an operating systemresponse to the alert, an alert silence button, an alert clear signal,or the like).

Method 800 may then conclude.

Several examples and embodiments of the apparatus and methods aredisclosed herein that include a variety of components, features, andfunctionalities. It will be understood that the various examples andembodiments of the apparatus and methods disclosed in the presentdisclosure may include any of the components, features, andfunctionalities of any of the other examples and embodiments of theapparatus and methods disclosed in the present disclosure in anycombination, and all of such possibilities are intended to be within thespirit and scope of the present disclosure.

Having the benefit of the teachings presented in the foregoingdescription and the associated drawings, many modifications of thedisclosed subject matter will become apparent to one skilled in the artto which this disclosure pertains. Therefore, it is to be understoodthat the disclosure is not to be limited to the specific examples andembodiments provided and that modifications thereof are intended to bewithin the scope of the appended claims. Moreover, although theforegoing disclosure and the associated drawings describe certainillustrative combinations of elements and/or functions, it will beappreciated that different combinations of elements and/or functions maybe realized without departing from the scope of the appended claims.

1. A levered landing gear, comprising: a first shock strut having alongitudinal axis, the first shock strut including first stop pad havinga first contact surface; a second shock strut disposed concentricallywith the first shock strut along the longitudinal axis such that thefirst shock strut and the second shock strut extend along a sharedextension axis; a truck lever coupled to the first shock strut and thesecond shock strut such that the second shock strut pivots the trucklever relative to the first shock strut, the truck lever including asecond stop pad having a second contact surface, wherein the truck leverpivots between an extended position and a static position relative tothe first shock strut, and wherein the first contact surface makesphysical contact with the second contact surface when the truck lever isin the static position to form a stop joint for the levered landinggear; and a first sensor disposed with the levered landing gear andconfigured to measure a first condition of the stop joint.
 2. Thelevered landing gear of claim 1, wherein the first sensor is a cameradisposed of in the first stop pad, and wherein the first condition ofthe stop joint measured by the camera includes at least one of: a debrispresence in the stop joint and a surface condition of the second contactsurface.
 3. The levered landing gear of claim 2, wherein the camera is athree-dimensional camera mounted in first stop pad via a camera actuatormember and includes a light oriented to shine on the second stop padwhile the levered landing gear is in the extended position.
 4. Thelevered landing gear of claim 1, wherein the first sensor is a gapsensor disposed of in the first stop pad, and wherein the firstcondition of the stop joint measured by the gap sensor includes a gapdistance between at least a portion of the first contact surface and atleast a portion of the second contact surface.
 5. The levered landinggear of claim 4, wherein the gap sensor is an eddy current sensormounted in a fixed position within the first stop pad.
 6. The leveredlanding gear of claim 1, wherein the first sensor is a thickness sensordisposed of in the second stop pad, and wherein the first condition ofthe stop joint measured by the thickness sensor includes a thickness ofthe second stop pad.
 7. The levered landing gear of claim 6, wherein thethickness sensor is an ultrasound sensor held in contact with a secondmounting surface of the stop pad via an actuator member, the secondmounting surface opposite to the second contact surface of the secondstop pad.
 8. The levered landing gear of claim of 1, further comprising:a second sensor disposed with the levered landing gear that measures asecond condition of the stop joint.
 9. The levered landing gear of claimof 8, wherein the first sensor is a camera mounted on the first shockstrut, and wherein the first condition of the stop joint measured by thecamera includes at least one of: a debris presence in the stop joint anda surface condition of the second contact surface; and wherein thesecond sensor is a gap sensor disposed of in the first stop pad, andwherein the second condition of the stop joint measured by the gapsensor includes a gap distance between at least a portion of the firstcontact surface and at least a portion of the second contact surface.10. The levered landing gear of claim 8, wherein the first sensor is acamera disposed of in the first stop pad, and wherein the firstcondition of the stop joint measured by the camera includes at least oneof: a debris presence in the stop joint and a surface condition of thesecond contact surface; and wherein the second sensor is a thicknesssensor disposed of in the second stop pad, and wherein the secondcondition of the stop joint measured by the thickness sensor includes athickness of the second stop pad.
 11. The levered landing gear of claim8, further comprising a computing device in communication with the firstsensor that compares the measured condition against a threshold and, inresponse to the measured condition satisfying the threshold, generatesan alert associated with the measured condition.
 12. A stop pad in alevered landing gear, comprising: a first contact surface, disposed ofin the levered landing gear relative to a second contact surface of thelevered landing gear so as to form a stop joint for a static position ofthe levered landing gear with the second contact surface; and a firstsensor disposed of within the stop pad, wherein the first sensormeasures a condition of the stop joint.
 13. The stop pad in the leveredlanding gear of claim 12, wherein the first sensor is a camera sensorfocused on the second contact surface, and wherein the condition of thestop joint is measured according to a visual appearance of the secondcontact surface.
 14. The stop pad in the levered landing gear of claim12, wherein the first contact surface is defined by a truck pad of thelevered landing gear, wherein the first sensor is a gap sensor, andwherein the condition is a gap distance between the first contactsurface and the second contact surface while the levered landing gear isin a static position.
 15. The stop pad in the levered landing gear ofclaim 14, wherein a second sensor of a camera sensor is focused on thesecond contact surface defined in a strut pad of the levered landinggear, wherein the second sensor is signaled to capture of an image ofthe second contact surface while the levered landing gear is in anextended position and correlate the image with the gap distance measuredwhile the levered landing gear is in the static position.
 16. The stoppad in the levered landing gear of claim 12, wherein the first contactsurface is defined in a truck pad of the levered landing gear, whereinthe first sensor is a thickness sensor, and wherein the condition is apad thickness of the truck pad.
 17. The stop pad in the levered landinggear of claim 16, wherein a second sensor of a camera sensor is focusedon the first contact surface, wherein the second sensor is signaled tocapture of an image of the first contact surface while the leveredlanding gear is in an extended position and correlate the image with thepad thickness measured by the thickness sensor.
 18. A method formonitoring the health of a levered landing gear, comprising: calibratinga sensor for measuring a condition of a stop joint formed by a firststop pad and a second stop pad of the levered landing gear against anominal condition of at least one of the first stop pad and the secondstop pad; monitoring, by the sensor, a current condition of the at leastone of the first stop pad and the second stop pad from the nominalcondition; determining whether a non-conformance from the nominalcondition of the at least one of the first stop pad and the second stoppad has been detected by the sensor for the current condition; and inresponse to determining that the non-conformance has been detected,generating an alert.
 19. The method of claim 18, further comprising, inresponse to determining that the non-conformance has been detected:capturing an image via a camera focused on the at least one of the firststop pad and the second stop pad; storing the image for retrieval tocorroborate the determination that the non-conformance has beendetected; and displaying the image to a user in response to the useracknowledging the alert.
 20. The method of claim 18, wherein thecondition includes at least one of: a thickness of the second stop padmeasured by the sensor via ultrasound; a gap distance between the firstsurface and the second surface measured by the sensor via an eddycurrent; a gap distance between the first surface and the second surfacemeasured by the sensor via range finding; a visual indication of debrisin the stop joint measured by the sensor via image recognition; a visualindication of corrosion in the stop joint measured by the sensor viaimage recognition; and a visual indication of surface marring in thestop joint measured by the sensor via image recognition.